Double-walled vacuum insulated containers for the storage of cryogenic liquids, e.g. liquid nitrogen, oxygen, helium and hydrogen are well known in the art. Since such cryogenic liquids boil at very low temperatures, for example liquid oxygen at -183.degree. C., liquid nitrogen at -195.8.degree. C., liquid hydrogen at -252.9.degree. C., liquid helium at -268.9.degree. C., even small quantities of thermal energy flowing from ambient into the cryogen cause significant losses of cryogen through evaporation.
The prior art has developed many thermal insulations to be disposed in the vacuum space of such double-walled containers to decrease the heat flow from ambient into the cryogen. Containers which represent the most advanced state of the art include thermal insulation systems comprising three basic components:
a vacuum space formed by double-walled construction;
a multilayered insulation comprised of layers of low conductive materials alternating with sheets of thin, flexible radiation barriers disposed within the vacuum space; and
heat-exchanger shields disposed within the multilayered insulation and connected to the necktube of the double-walled container to conduct part of the inflowing thermal energy through the necktube wall into the cold effluent gas which carries it away to the atmosphere. Such an insulation system is described, for example, in U.S. Pat. No. 3,133,422--Paivanas et.al.
The prior art has made many improvements in the three basic components of such thermal insulations and their optimization as a system. However, the improvements to these basic components and their optimization as a system are approaching the limit of efficiency beyond which further improvements result in negligible advantage or are not economically feasible.
Other prior art methods to increase thermal insulation effectiveness include placing a shield between the hot and cold walls of the double-walled container, which shield is cooled by a vaporizable liquid. Such a shield intercepts heat flowing from the warm to cold wall which heat is absorbed by the vaporizing liquid and is "rejected" as vapor back to the warm side of the system. However, such systems are structurally complex, require an auxiliary refrigerant fluid and a source for replenishing the refrigerant fluid. Thus such systems are expensive and the auxiliary equipment adds bulk to the system.
The need therefore exists for a container for the storage of cryogenic liquids having an improved thermal insulation system which is effective, structurally simple, inexpensive, and does not add undue bulk to the container.
A container for the storage of cryogenic liquids having an improved thermal insulation system would result in cost savings in reducing the loss of the cryogenic liquid due to evaporation. Since the loss of cryogenic liquid due to evaporation is decreased, the holding time for a given container--i.e. the time interval in which a user of cryogenic liquid would have to have a container refilled to replenish losses due to evaporation--would be increased. Thus the frequency of refilling the cryogen would be reduced.